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We report initial results from the far-infrared fine structure lineobservations of a sample of 44 local starbursts, Seyfert galaxies and infraredluminous galaxies obtained with the PACS spectrometer on board Herschel. Weshow that the ratio between the far-infrared luminosity and the molecular gasmass, Lfir/MH2, is a much better proxy for the relative brightness of thefar-infrared lines than Lfir alone. Galaxies with high Lfir/MH2 ratios tend tohave weaker fine structure lines relative to their far-infrared continuum thangalaxies with Lfir/MH2 < 80 Lsun/Msun. A deficit of the [C II] 158 micron linerelative to Lfir was previously found with the ISO satellite, but now we showfor the first time that this is a general aspect of all far-infrared finestructure lines, regardless of their origin in the ionized or neutral phase ofthe interstellar medium. The Lfir/MH2 value where these line deficits start tomanifest is similar to the limit that separates between the two modes of starformation recently found in galaxies on the basis of studies of their gas-starformation relations. Our finding that the properties of the interstellar mediumare also significantly different in these regimes provides independent supportfor the different star forming relations in normal disk galaxies and majormerger systems. We use the spectral synthesis code Cloudy to model the emissionof the lines. The expected increase of the ionization parameter with Lfir/MH2can simultaneously explain the line deficits in the [C II], [N II] and [O I]lines.

Far-Infrared Line Deficits in Galaxies with Extreme Lfir/MH2 Ratios